[I] Necrosis
Necrosis is the commonest and destructive type of effect. These kinds of symptoms are produced by rather unspecialized parasites. Among these pathogens, the most destructive ones, Pythium, Botrytis, and Fusarium causing wilts, and many others, cause immediate and severe damage to host tissues and obtain their nutrients from cells killed in advance by secretion of enzymes and toxins.
They are thus called necrotrophs. Some other necrotrophs, a somewhat intermediate group between the abovesaid type and the specialized parasites such as rust, mildews, etc., do not kill cells in advance but they invade and kill host cells. Such necrotrophs include fungi like Phytophthora, Venturia, Tapering, and Claviceps. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Necrosis is caused by specialized parasites also like rusts, mildews, and smuts but only in later stages of attack when they sporulate. Thus during the early stages of infection, there is only a slight adverse effect. Necrosis is also caused by bacteria and viruses.
The symptoms which are of necrotic nature are as follows:
1. Leaf spots. The cells are killed in a limited area to form lesions or spots. The spots are of various sizes and shapes as zonate, target board, angular, and frog eye. Tar spots are a characteristic type of leaf spots that appear as raised, black-coated fungus bodies giving the appearance of a flat drop of tar on leaves. Example = Rhytisima and Phyllachora. In some cases, the dead tissue shrinks and separates from the healthy tissue. This condition is known as a shot hole. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
2. Streaks or stripes. These are elongated, narrow lesions on leaves, which are usually of brown shade. Example: barley stripe by Helminthosporium.
3. Blights. This gives a burnt appearance. There is the sudden death of plant parts such as leaves, blossoms, stamen, twigs, or even entire plants. The dead part usually turns brown or black and soon disintegrates. Example-early and late blight of potatoes, some bacteria such as Xanthomonas.
4. Damping off. This is the condition where the stem is attacked near the ground level. The affected portion becomes constricted and weak, and finally disintegrates causing the seedling to topple down and die. Damping off of vegetable seedlings is a common example caused by Pythium and Phytophthora. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
5. Burn, scald or scorch. In succulent organs like fruits, limited areas die and turn brown.
6. Rots. The infected tissue dies, decomposes rapidly, and turns brown. Fungi and bacteria which are able to dissolve cell walls cause this symptom. The rot may be root rot, leaf rot, stem rot, bud rot, or fruit rot.
7. Wilting. Wilting of infected leaves due perhaps to plugging of xylem vessels by fungus or mucilaginous substances is caused by some fungi like Fusarium spp. and Trichometasphaeria turcica (in maize). Later, the whole plant wilts and dies.
8. Die-back. Death of plant parts, as stem or branches from tip backward.
9. Cankers. A canker is a dead area in the bark or cortex of woody stems. They are often large areas with definite margins. Dead bark splits and falls away. Example = citrus cake (bacterial).
10. Chlorosis. Discoloration from normal colour is common in some cases. The green pigment may be destroyed and the tissue becomes yellow. This condition known as chlorosis is caused by viruses, bacteria, and some fungi.
11. Blotch. Some fruits, due to infection, develop superficial growth appearing as a blotch area. It occurs in the sooty blotch and flyspeck disease of apple.
12. Scab. This is a rough, crust-like lesion or a freckled organ. This mostly occurs in fruits and vegetables. Example – apple scab by Venturia inaequalis.
13. White blisters or pustules. On the leaves of some plants there develop numerous shining white blister-like pustules which break open at maturity exposing a powdery mass of spores. Such diseases are commonly called white rusts caused by Albugo spp.
14. Smuts. This refers to a sooty or charcoal-like powdery mass. In plants infected by smuts (Ustilago. Tilleria), this is very common. Smut spores develop mostly in floral organs, particularly ovaries, though also on leaves, stems, and roots.
15. Rusts. The rusts appear as small pustules of spores forming a powdery mass of brownish, red, yellow, or dark brown colour. This breakthrough epidermis. Example – members of Uredinales.
16. Mildews. In this case, the pathogens grow mostly superficially on the host surface. They appear as white, grey, brownish, or purple patches of various sizes on leaves. In the case of downy mildews (Peronosporaceae), the superficial growth is a tangled cottony or downy layer consisting of the sporangiophores and sporangia of the pathogen. In powdery mildews (Erysiphaceae), there is superficial, white powdery growth of mycelium, numerous conidiophores, and conidia. These give a dusty appearance.
[II] Hypertrophy and Hyperplasia (over-development)
In some diseases, there is an abnormal increase in the size of an organ or of an entire plant. This increase may be due to an increase in the size of individual cells of affected tissue i.e., hypertrophy, or due to an increase in the number of cells as a result of cell division i.e., hyperplasia. Various overgrowths developed are of the following types. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
1. Elongated internodes. Some infected plants develop elongated internodes and become abnormally tall. Example=rice infected with Gibberella Jujikuroi, Euphorbia cyperissias with Uromyces pisi, sugarcane with Sclerospora saccharin.
2. Galls and tumors. These are globose, elongated, or irregular large-sized out-growths formed on the attacked part. Smaller galls are warts tubercles. Examples are club-root of crucifers, some species of Albugo, crown-gall, and root-knot.
3. Witche’s broom. They are formed by fungi (Taphrina, some rusts), bacteria (Corynebacterium fascines), and certain viruses as well as insects and mites. They arise basically from the stimulation of structures that normally remain dormant, as buds. The broom very often appears as an upright cluster of small shoots, contrasting with the horizontal growth habit of normal shoots.
4. Curls. In some diseases, the leaves are arched, puckered, twisted, curled, and distorted. Examples- are peach leaf curl (Taphrina deformans), and papaya leaf curl (Virus).
5. Floral abnormalities. Some fungi such as Albugo candida cause the infected inflorescence to enlarge, green and fleshy with stamens converted into leafy structures. The inflorescence becomes distorted.
[III] Hypoplasia (under-development)
In these instances, there is reduced development of the whole plant, parts of the plant certain tissues, flowers or fruit (sterility), or chlorophyll (chlorosis). In extreme cases, it leads to atrophy in which the organ or tissue does not develop at all.
1. Chlorosis. Reduced development of chlorophyll results in various kinds of chlorosis, mosaic, and mottling in many viral and deficiency diseases. Chlorosis may take the form of streaking. There may be vein clearing- a translucent appearance of the veins or vein banding in which dark green bands bound the veins or general chlorosis of the whole leaves as in yellows.
2. Reduction of the individual organs. Individual leaves and flowers may be reduced in size and/or altered in shape. Reduced leaves develop in viral infections and in some fungi such as Exobasidium. Examples are “fern leaf”, where lamina disappears, ‘little leaf’ etc. Internodes are reduced in dwarf bunt of wheat caused by Tilletia co traversa, and in several viral diseases.
3. Floral abnormalities. In another smut of Caryophyllaceae, stamens become sterile (caused by Ustilago violacea). One very common and characteristic floral abnormality is phyllody i.e. transformation of floral parts into green leafy twisted structures. These are caused by mites, insects, fungi, and viruses. Example-green ear disease of bajra.
Stages in Disease Development: Disease Cycle
In an infectious disease, there is a series of more or less distinct events which occur in sequence and lead to the development and perpetuation of the disease and the pathogen. This chain of events is called a disease cycle. The disease cycle involves the changes in the plant and the plant symptoms as well as those in the pathogens and spans periods within a growing season and from one growing season to the next. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The main events of a disease cycle include:
(i) inoculation, (ii) prepenetration, (iii) penetration, (iv) infection (also includes invasion), (v) growth and reproduction of the pathogen, (vi) dissemination of the pathogen, and (vii) seasonal carry-over of pathogen.
1. Inoculation. Inoculation is the contact of a pathogen with a plant. This is the inoculum that lands on or is otherwise brought into contact with the plant. Inoculum is any part of the pathogen that can cause infection.
In fungi, inoculum may be spores sclerotia, or mycelial fragments. In bacteria, mycoplasmas, viruses, and viroids, the inoculum is always whole individuals. In nematodes, inoculum may be adults, larvae, or eggs, whereas in parasitic higher plants it is plant fragment or seed.
The inoculum that survives the off-season periods (winter or summer) and causes the original infection in the growing season is called primary inoculum, and the infections as primary infections. Inoculum produced from these primary infections that actually spread the disease in the field under favourable conditions is called secondary inoculum, which brings about secondary infections.
Inoculum in the absence of its host from the field survives in plant debris, field soil, seed, tubers, transplants or other plant parts, perennial weeds, and alternate hosts. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The inoculum is carried to host plants and this landing or arrival of inoculum is passive by wind, water, insects, etc, or in some cases also by active growth as in some root-infecting fungi like Armillaria mellea.
2. Prepenetration. This phase includes all the events prior to the actual entry of the pathogen. Such events include (i) germination of spores and seeds, (ii) hatching of eggs (nematodes), (iii) attachment of pathogen to host, and (iv) recognition between host and pathogen (early event-not still understood clearly). Lack of specific recognition factors in plant surface may not allow the attachment of pathogen to it. Such factors in plants include lectins (proteins or glycoproteins) and some oligo-and polysaccharides. In viral pathogens, lack of recognition of its nucleic and host ribosomes may lead to failure in disease.
3. Penetration. This is the actual entry of the pathogen into its host plants. Pathogens penetrate plant surfaces in different ways:
(i) Direct penetration through intact plant surfaces. (ii) Through natural openings, and (iii) Through wounds.
4. Infection. This is the process by which a pathogen establishes contact with host cells or tissues and procures nutrients from them. This stage also includes invasion and to some extent growth and reproduction of the pathogen. During the invasion, the pathogens colonize the host tissues in different ways and to different extents. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The time elapsing between penetration or more accurately spore germination and established infection is called the infection period. We shall describe this stage in detail later. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
5. Growth and reproduction of pathogen. Pathogens invade and infect tissues by growing and multiplying into them. In this way, they colonize and infect more areas or parts of the attacked plant. The period between infection, or more accurately spore germination, and the appearance of visible symptoms is called the incubation period. Thus incubation period includes the full life cycle of the pathogen. It may thus be seen that between spore germination and complete expression of the disease (symptoms), a series of events in the host.
This chain of events between the time of infection, or more accurately spore germination, and the complete expression of disease is called the disease cycle or disease development. For the spread of secondary inocula to perpetuate the disease in the field during the growing season of plants, dissemination is also sometimes included as a stage in disease development. Seasonal carry-over of the inoculum to the next season is also similarly included by some under-disease development.
6. Dissemination of pathogen. After the pathogen has grown and multiplied in or on the infected host, it spreads to new, healthy plants. Dissemination is the transfer of inoculum from the site of its production to the susceptible host surface. Some pathogens disperse in an active manner, whereas most are passive with the help of an agent of dispersal. The chief agents of dissemination are:
(i) Air, (ii) Water, (iii) Vectors i.e. insects, mites, nematodes, etc. (iv) Man.
7. Seasonal carry-over of the pathogen. In the absence of their hosts, the pathogens undergo over-summering or over-wintering. Pathogens have evolved a great variety of means of this seasonal carry-over.
At the onset of suitable conditions in the next growing season, these resting structures become active and produce inoculum. This inoculum then is taken to the host surface. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Pathogens
A pathogen is a living entity that can cause disease. Some authors use the term in a wider sense to include also non-living causes such as mineral deficiencies excesses in soil and other physical factors etc. that bring about physiological disorders rather than diseases in plants. They define a pathogen, therefore, as an entity that incites disease.
However, if diseases are caused, the pathogen should include only living entities. The pathogens that attack plants belong to different taxonomic groups of microorganisms and there is a wide range in their size. Following are the different types of pathogens that cause disease in plants. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
(i) Fungi, (ii) Bacteria, (iii) Mycoplasmas, (iv) Viruses and Viroids. (v) Nematodes (vi) Protozoa, and (vii) Parasitic higher plants.
These pathogens cause infectious diseases in plants. Such diseases are characterized by the ability of the pathogen to grow and multiply rapidly on diseased plants and also by its ability to spread from diseased to healthy plants and, thereby, cause new infections.
Parasite versus Pathogen. An organism that lives on or in some other organism and obtains its food from the latter is called a parasite. The relationship between a parasite and its host is called parasitism. A plant parasite is an organism that becomes intimately associated with a plant and multiplies or grows at the expense of the plant.
The removal of nutrients and water from the host by the parasite usually leads to reduced efficiency in the normal growth of the plant and may become detrimental to its further development and reproduction. Thus in many situations, parasitism is intimately associated with pathogenicity (ability to cause disease), since a pathogen must first be able to invade and become established in the nose tissue.
Does this mere removal of nutrients from the host by a pathogen cause damage reaching levels responsible for a disease? In most plant diseases, however, the amount of damage caused to plants is often much greater than would be expected from the mere removal of nutrients by the parasite.” This additional damage results from the substances secreted by the pathogen or produced by the host in response to stimuli originating in the pathogen. Such substances have adverse effects on plant physiology.
These substances include enzymes, toxins, and growth regulators. Thus pathogens interfere with the essential functions of the plant, a parasite normally does not. A pathogen is thus a parasite plus something more. All pathogens are parasites but not all parasites are pathogens. Thus pathogen’s ability to cause disease resides in its ability to cause damage to plants in addition to the removal of nutrients from it. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
The extent of damage caused to plants depends on the level of specialization in a pathogen. The least specialized pathogens cause maximum damage whereas the most specialized ones cause minimum damage to their host plants.
Inoculum Potential
Inoculum is that part of a pathogen that is transmitted to or contacts a host and is capable of infecting the host. In fungi, inoculum may be mycelial fragments, spores, (asexual as well as sexual), rhizomorphs, or sclerotia. In bacteria, mycoplasma, viruses, and viroids, the inoculum is whole individuals; in nematodes, inoculum may be adults, larvae, or eggs whereas in parasitic higher plants inoculum may be seeds or plant fragments. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
As a result of dissemination from the site of its production, the inoculum reaches the surface of its host plant. Will this inoculum be able to infect the host and cause disease? There are many factors that determine whether the pathogen will be successful in entering the plant.
One way of measuring whether a pathogen will be successful in infecting a plant and causing disease is to describe the inoculum potential of a particular pathogen. This term was coined by J.G. Horsfall in 1932. Since then the term has been heatedly discussed, and even today there is no general agreement as to what we mean when we talk about inoculum potential.
Some pathogens require only a few spores to cause successful infection, whereas others may require a larger number of spores – several hundred or several thousand. Sometimes in spite of the presence of an increasing number of propagules on plant surface infection does not occur and there is no disease development. It means factors other than the number of infectious particles, are also involved in disease development. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Horsfall (1932) used the term inoculum potential to denote “the number of infective particles present in the environment of the uninfected host.” This essentially is what we now refer to as inoculum density. Since the origin of this term, other plant pathologists have broadened the scope of the factors that are concerned with inoculum potential. For instance, the term was elaborated by Garrett (1956) as “the energy of growth of a parasite available for infection of a host at the surface of the host organ to be infected”.
A detailed discussion was given by Garrett (1960) and Wood (1967). The concept of inoculum potential was first used for soil-borne pathogens and has been applied by Garrett (1956, 1960) to mycelial strands and rhizomorphs.
Inoculum potential is an important factor in infection. Dimond and Horsfall (1960, 1965) used this term in the sense of the number of spores produced by a population of diseased plants. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
They described it as “the number of independent infections that are likely to occur in a given situation in a population of susceptible healthy tissues,” and thus considered it as the result of environmental effects, the infective ability of the pathogen, the susceptibility of the plant, and the amount of inoculum. Inoculum potential thus includes not only the number of infecting propagules per unit area of host tissue but also their aggressiveness. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
As visualized now, inoculum potential is “a measure of the biological enc available for the colonization of a host.” It is therefore a function of the Tom (i) inoculum density, (ii) the nutrients available to the infectious units in them to germinate or grow, (iii) the environment, (iv) the virulence on the capacity of the organism to cause disease, and (v) the susceptibility (to maintain a favourable nutritional status for the infecting pathogens in one or more of these factors can alter the inoculum potential.
Dissemination of Pathogens
Dissemination is what happens between the take-off of the inoculum and its deposition on the host surface.
After successful liberation from the perennating structure, the pathogens are transmitted from the site of their production to an area where a susceptible host is growing. Transmission may occur at short or long distances. Butler recognized it as follows:
1. Continuous dissemination. It occurs at short distances and is related to the dispersal of pathogens in a given geographical unit. This occurs normally from host to host, field to field, or locality to locality, closely connected by geography.
2. Discontinuous dissemination. It involves a long jump transmission, ordinarily between two geographical units separated by physical barriers such as oceans, mountains, deserts, etc. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Usually, the dispersal of inoculum is a passive process occurring with the help of external agents. However, there are some inocula that disperse themselves actively by means of zoospores, motile cells, etc. Accordingly, dispersal may be (i) Autonomous, or (ii) Passive.
Autonomous dissemination. This is the active dispersal of inoculum. It is a normal practice in crop husbandry and distribution of plants and plant products that pathogens causing diseases of fruits and vegetables disseminate through the agency of soil, seeds, or plant parts. They do not disperse through the intervention of any external agency such as insects, wind, water, etc.
The contaminated soil is carried with farm implements and machinery and through the boots of workers. The contaminated seed also carries pathogens such as smuts. Plant parts used as planting stocks may be contaminated. Infected plant debris may also be carried along with farm implement as tractors, tillers, etc. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
A few pathogens such as nematodes, fungal zoospores, and bacteria can move very short distances on their own from one plant to another. Fungal hyphae and rhizomorphs can grow actively between tissues in contact and sometimes through the soil toward nearby roots. The spores and seeds of some higher plants are discharged forcibly. Except for these cases, in most cases, dissemination is a passive process.
Passive dissemination. This is the general and common way of dispersal of pathogens. The main agents of dissemination are : (i) Wind (ii) Water (iii) Insects (iv) Other animals, (v) Man, and (vi) Fungi.
1. Dissemination by the wind. Most fungal pathogens attack aerial parts of plants and the seeds of most parasitic plants are dispersed by air. Fungi have relatively small spores, which differ widely in their form and method of formation. Various forms of inocula in this type of pathogen are conidia, uredospores of rust, basidiospores, ascospores, etc.
Spores are produced in different ways in different kinds of structures. They are formed either in simple sporangia or on complex conidiophores by budding or fission. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Dispersal by air occurs mainly by external forces of physical nature and thus subject is of a physical rather than biological nature. Uredospores of Puccinia graminis fly at about 10,000 Km. They may spread at a distance of about more than 3,000 km. in only two months.
Phytophthora infestans and Puccinia malvacearum also spread at long distances. A detailed account of the dispersal of airborne pathogens is given by Ingold (1971) and Gregory and Monteith (1967). (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Bacteria and nematodes present in soil may be blown away along with soil particles in the dust. Wind also helps in the dissemination of bacteria, nematodes, and fungal spores by blowing away rain splash droplets containing these pathogens. Wind causes adjacent plants or plant parts to rub together. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
2. Dissemination by water. Water as water splash is an important method of liberation and dispersal of fungal spores. Coffee rust, cells of many bacteria, and slimy spores of fungi like Septoria, Fusarium, and Colletotrichum are dispersed by water splash (a combination of water and air). (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Water itself may also be an agent of dispersal in propagules of aquatic fungi and soil-inhabiting fungi. Plant parts infected with bacteria such as Xanthomonas malvacearum are carried by water. Plasmodiophora brassicae also disperses in this way.
Water is important in three ways: (i) Bacteria, nematodes, and spores, sclerotia, and mycelial fragments of fungi present in soil are disseminated by rain or irrigation water that moves on the surface or through the soil, (ii) All bacteria and spores of many fungi are exuded in a sticky liquid and depend for their dissemination on rain or (overhead) irrigation water which either washes them downward or splashes them in all directions. (iii) Raindrops or drops from overhead irrigation pick up the fungal spores and any bacteria present in the air and wash them downward where some of them may land on plants.
3. Insects. Insects, particularly aphids and leafhoppers, are the most important vectors of viruses and mycoplasma. This has reached its climax in viruses, though also important in the dispersal of bacterial and fungal pathogens.
Among bacteria, which may be carried externally or internally on insects are, Erwinia amylovora (cause of blight of pear and apple) carried by flies, aphids, ants, beetles, wasps, and bees: Xanthomonas Stewart (bacterial blight of maize) by flea beetles: Pseudomo savastoni (olive knot) by an olive fly. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Dispersal of fungal pathogens by insects is well-known. They feed on infected plants and move. About 70 species of pathogenic fungi are transmitted by more than 100 species of insects belonging to at least six orders.
Spores of Ustilago violacea and Botrytis anthophila are disseminated by pollinating insects, conidia of Claviceps by honey-dew feeding insects; spores of Ceratocsytis fagacearum (oak wilt) by beetles; hyphae and conidia of Stereum sp by siricid; Ceratocystis ulmi (cause of Dutch elm disease) grows within eggs of beetles (Scolytus), sticky pycnospores of Endothia parasitica by insects, birds, etc.
4. Mites. Some viruses are disseminated by mites. Nigrospora oryzae causing wheat diseases is carried by a mite, Pediculopsis. Fusarium and Sporotrichum are also seen associated with mites. They carry some viruses internally and bacteria and fungal spores externally.
5. Nematodes. Some viruses, bacteria, and fungal pathogens are carried by nematodes. These are present internally as well as externally. Corynebacterium fascines (leafy galls of herbs) by an ectoparasitic nematode, Aphelenchoides. Dilophospora alopeuri is carried by Anguina tritici. Fusarium oxysporum vasinfectum (cottom wilt) by root-knot nematodes.
6. Other animals. Some small or large animals while moving through the crop area and touching the infected plant, carry the pathogens. Conidia of Mycosphaerella linoleum can be carried by spiders, mice, frogs, birds, and dogs which move on wet plants. Woodpeckers carry spores of tree pathogens. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
7. Man. Man is an important agent responsible for dispersal within crop areas, within the land mass (continental dispersal), or between land masses (intercontinental) dispersal. Within the field, it occurs through handling, contaminated tools, and transport of contaminated soil on feet, equipment, etc. (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Following are some examples of pathogens introduced by man. Striga Asiatica (root parasite of maize, sorghum, and other cereals) was introduced in the U.S.A. on seeds imported from Asia or Africa. Endothia parasitica (chestnut canker) was brought in from the Orient.
The coffee rust (Hemileia vastatrix) present in Africa and Asia, was introduced to coffee in Brazil. Phytophthora infestans. Uncinula necator, Plasmopora viticola, Puccinia polyspora, and Pseudoperonospora humuli have spread from the New to Old World. Despite many inspection and quarantine measures, several pathogens have entered different countries through man.
8. Fungi. There are reports of about half-a-dozen viruses transmitted by fungi. The chytrid, Olpidium brassicae is found to transmit tobacco necrosis virus (TNV). (BSc 2nd Year Introduction to Plant Pathology Notes Study Material)
Olpidium also transmits lettuce big vein (Lt BV) and tobacco stunt virus (TSV). Potato virus X (PVX) is transmitted by Synchytrium, endobioticum, and the soilborne wheat mosaic virus by Polymyxa graminis, potato mop-top virus by Spongospora subterranea. For the relationship between viruses and fungi, see details given by Hollings and Stone (1969).
BSc 2nd Year Introduction to Plant Pathology Notes Study Material
BSc 2nd Year Sample Model Practice Mock Test Question Answer Papers